Likening the interactions between instrumental behavior and the environment to a feedback system holds the possibility of a comprehensive quantitative theory of performance. Such a theory can account for the differences in performance on interval and ratio schedules. It can apply to complex performances, such as those on concurrent schedules. It can apply both to positive reinforcement and aversive control. It requires a simple, but fundamental, change in the law of effect: from a law based on contiguity of events to a law based on correlation between events. The notion of correlation requires measures and units of analysis that transcend momentary events, extending through time. Much recent research suggests that behavior and its environmental consequences can be studied most fruitfully on such a molar level of analysis. In a wide variety of situations, behavior appears to depend upon variables like rate of reinforcement and rate of punishment, variables that must be averaged over a substantial time sample. The behavior that interacts with such molar independent variables can be viewed as similarly molar, that is, based on substantial periods of time, rather than countable discrete responses. Indeed, research with response-independent reinforcement has shown that in some situations time-based measures of behavior alone are sufficient to demonstrate lawful relations between behavior and rate of reinforcement.